Semiconductor memory device and method for producing the same

ABSTRACT

Disclosed is a non-volatile semiconductor memory device that uses a inversion layer provided on a semiconductor substrate as a data line. The memory device can reduce variation of characteristics among memory cells and can reduce bit cost. A plurality of assist gates are formed in the upper part of a p-type well through a gate oxide film. In the upper part of an interlayer insulator that covers those assist gates are formed word lines that are used as control electrodes. The width of those word lines is, for example, 0.1 μm, and each word line is separated from its adjacent word lines by a side wall spacer that is a silicon oxide film having a thickness of about 20 nm.

The present application claims priority from Japanese application JP2003-205695 filed on Aug. 4, 2003, the contents of which are herebyincorporated by reference as if set forth in the entirety herein.

FIELD OF THE INVENTION

The present invention relates to a semiconductor memory device and amethod for producing the same, and more particularly, to a techniqueapplied to non-volatile semiconductor memory devices, each of which usesa inversion layer formed on its semiconductor substrate as a data line.

BACKGROUND OF THE INVENTION

Recently, non-volatile semiconductor flash memories have come into wideuse as data storage memories with excellent portability. The per-bitprice of those flash memories has been rapidly dropping just from theirminiaturization. This per-bit price reduction has actually been achievedby the element structure improvement or employment of multi-bit storagesystems with respect to those flash memories.

Typical methods for forming memory arrays of large capacity flashmemories used for files are NAND type and AND type. In the NAND typememory, memory cells are connected serially. In the AND type memory,memory cells are connected in parallel. The AND type memory in whichmemory cells are disposed in parallel is usually considered to besuitable for multi-bit storage operations, since it enables controllingof the number of electrons stored in a floating gate. Additionally, theAND type memory employs a hot electron writing method, so that itswriting is fast. The NAND type is disclosed in “IEEE InternationalElectron Devices Meeting” (pp.775-778, 2000)” by F. Arai et al., whilethe AND type is disclosed in “IEEE International Electron DevicesMeeting (pp.29-32, 2001)” by T. Kobayashi et al.

The official gazette of JP-A 156275/2001 illustrates a non-volatilememory technique that achieves both requirements of an arrayconfiguration in which memory cells are connected in parallel and asmall memory cell region. This gazette further illustrates how to useeach inversion layer formed on a semiconductor substrate located underan assist gate as a line. Also illustrated by the official gazette ofJP-A No.2001-326288 is a technique for configuring a memory cell arrayat narrow word line pitches to achieve high density disposition ofmemory cells.

As described above, the AND type flash memory, which employs the hotelectron writing technique, is fast in writing. Because the hot electronwriting method employs source side injection, the method is alsoconsidered to be suitable for simultaneous writing in many memory cells.Additionally, because memory cells in an array are connected inparallel, each memory cell is not affected by the information stored inother adjacent memory cells so easily. This is why the AND type flashmemory is also considered to be suitable for multi-bit storage per cell.

In spite of such advantages, the AND type flash memory continues topresent difficulties. Because the AND type flash memory has an arraystructure in which diffusion layers are disposed in parallel, it isdifficult to reduce the line pitches that are parallel to data lines dueto the spread of the diffusion layers or existence of isolation regions.To solve this problem, a method for using inversion layers formed underthe electrodes disposed in parallel to the data lines as local datalines may enable the subject AND type flash memory to operate withoutdiffusion layers to be formed by impurity injection. This method isillustrated in the official gazette of JP-A No. 156275/2001.

However, each inversion layer usually has a resistance higher than thatof the diffusion layer formed by means of high density impurityinjection into the object semiconductor substrate. This is why the localdata line resistance is different among places in the memory array, sothat as the voltage falls, the potential to be applied to each targetmemory cell changes and the writing characteristic differs among memorycells significantly. This problem is accentuated as local data linesbecome longer. Another problem to arise from the employment of the abovedescribed memory structure is that if the flash memory is structured sothat local data lines are connected to a global data line at a shortdistance through a switch simply, the number of memory cells per localdata line is reduced and the area penalty of a selected transistorportion increases.

SUMMARY OF THE INVENTION

Under such circumstances, the present invention provides a technique forachieving the reduction of the variation of writing characteristicsamong memory cells, which depends on the place of each target memorycell in a memory cell array, and the reduction of the low bit cost in anon-volatile semiconductor memory device that uses inversion layersformed in the semiconductor substrate as data lines.

The present invention provides a technique for further miniaturizingmemory cells in a semiconductor memory device that uses inversion layersformed in the semiconductor substrate as data lines.

The novel features of the present invention will become more apparentfrom the description of this specification and the accompanyingdrawings. Typical aspects of those to be disclosed in this specificationwill be described briefly as follows.

In an embodiment of the present invention, the semiconductor memorydevice may include a plurality of assist gates formed on a main surfaceof a first conductor type semiconductor substrate through a firstinsulator and extended in a first direction of the main surface. Alsoincluded may be a plurality of word lines formed on the plurality ofassist gates through a second insulator and extended in a seconddirection that crosses the first direction, as well as a plurality ofmemory cells disposed at nodes of the plurality of assist gates and theplurality of word lines. The semiconductor memory device may employ amemory array structure in which a second conductor type inversion layerformed electrically on the surface of the semiconductor substratelocated under the plurality of assist gates may be used as a line forconnecting each of the plurality of memory cells to another. Theplurality of word lines may be separated electrically from each anotherthrough a side wall spacer that may be an insulator formed at the sidewall of each of even-numbered or odd-numbered word lines, and the spacebetween adjacent word lines may be determined to be ½ of the width ofthe word lines, thereby each local data line may be reduced in lengthwithout decreasing the number of memory cells per local data line.

BRIEF DESCRIPTION OF THE DRAWINGS

Understanding of the present invention will be facilitated byconsideration of the following detailed description of the preferredembodiments of the present invention taken in conjunction with theaccompanying drawings, in which like numerals refer to like parts, andwherein:

FIG. 1 is a schematic top view of a major portion of a memory cell arrayof a semiconductor memory device;

FIG. 2 is a cross sectional view of a semiconductor substrate along anA-B line in FIG. 1;

FIG. 3 is another cross sectional view of the semiconductor substratealong a C-D line (cross sectional direction of word lines) in FIG. 1;

FIG. 4 is still another cross sectional view of the semiconductorsubstrate along an E-F line (cross sectional direction of assist gates)in FIG. 1;

FIG. 6 is a circuit diagram of an equivalent circuit of thesemiconductor memory device;

FIG. 7 is still another cross sectional view of the semiconductorsubstrate along a G-H;

FIG. 8 is a cross sectional view of the memory cell array of thesemiconductor memory device;

FIG. 9 is a table on correspondence among multi-bit information, writingword lines and threshold values used for the semiconductor memorydevice;

FIG. 10 is a block diagram of the semiconductor memory device;

FIG. 11 is another block diagram of the semiconductor memory;

FIG. 12 is a top view of a memory mat for describing how to manufacturethe semiconductor memory device;

FIG. 13 is a cross sectional view of a major portion of the memory matfor describing how to manufacture the semiconductor memory device;

FIG. 14 is a top view of the memory mat for describing how tomanufacture the semiconductor memory device;

FIG. 15 is a cross sectional view of a major portion of the memory matfor describing how to manufacture the semiconductor memory device;

FIG. 16 of is another cross sectional view of the major portion of thememory mat for describing how to manufacture the semiconductor memorydevice;

FIG. 17 is still another cross sectional view of the major portion ofthe memory mat for describing how to manufacture the semiconductormemory device

FIG. 18 is a top view of the memory mat for describing how tomanufacture the semiconductor memory device;

FIG. 19 is still another cross sectional view of the major portion ofthe memory mat for describing how to manufacture the semiconductormemory device;

FIG. 20 is still another cross sectional view of the major portion ofthe memory mat for describing how to manufacture the semiconductormemory device;

FIG. 21 is another top view of the memory mat for describing how tomanufacture the semiconductor memory device;

FIG. 22 is a cross sectional view of a major portion of thesemiconductor substrate of the semiconductor memory device;

FIG. 23 is a circuit diagram of an equivalent circuit when in writinginformation in the semiconductor memory device;

FIG. 24 is a schematic top view of a major portion of contact regionsprovided at both ends of a memory mat of a semiconductor memory device;

FIG. 25 is a schematic chart for describing a relationship between dummycells and word lines in the semiconductor memory device;

FIG. 26 is a cross sectional view of a major portion of a semiconductorsubstrate of the semiconductor memory device;

FIG. 27 is a schematic top view of a major portion of the memory mat ofthe semiconductor memory device;

FIG. 28 is a circuit diagram of the equivalent circuit when in writinginformation in the semiconductor memory device;

FIG. 29 is a circuit diagram of the equivalent circuit when in writinginformation in the semiconductor memory device;

FIG. 30 is a cross sectional view of a major portion of thesemiconductor substrate of the semiconductor memory device;

FIG. 31 is another cross sectional view of the major portion of thesemiconductor substrate of the semiconductor memory device; and

FIG. 32 is still another cross sectional view of the major portion ofthe semiconductor substrate of the semiconductor memory device.

DETAILED DESCRIPTION

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the present invention, while eliminating,for the purpose of clarity, many other elements found in typical turfand soil management systems and methods of using the same. Those ofordinary skill in the art may recognize that other elements and/or stepsare desirable and/or required in implementing the present invention.However, because such elements and steps are well known in the art, andbecause they do not facilitate a better understanding of the presentinvention, a discussion of such elements and steps is not providedherein. The disclosure herein is directed to all such variations andmodifications to such elements and methods known to those skilled in theart.

One embodiment of the present invention may be illustrated in FIGS. 1-5.FIG. 1 is a schematic top view of a major portion of a semiconductorsubstrate for describing a memory array of a flash memory. FIG. 2 is across sectional view of the semiconductor substrate along an A-B line(cross sectional direction of assist gates) shown in FIG. 1. FIG. 3 isanother cross sectional view of the semiconductor substrate along a C-Dline (cross sectional direction of word lines) shown in FIG. 1. FIG. 4is still another cross sectional view of the semiconductor substratealong an E-F line (cross sectional direction of assist gates) shown inFIG. 1. FIG. 5 is a circuit diagram of an equivalent circuit of thememory array. In those drawings, metallic wiring, etc. except fornecessary portions for the description are omitted.

An n-type well 3 may be formed in a semiconductor substrate(hereinafter, also termed a substrate) 1 made of p-type single-crystalsilicon, and a p-type well 4 may be formed inside the n-type well 3(3-layer well structure). As shown in FIG. 1, the flash memory in thisfirst embodiment might not have an isolation region on the substrate 1of the memory cell array. The flash memory might not have a MISFETdiffusion layer (source and drain), which is usually formed usually bymeans of high density impurity injection.

In the upper part of the p-type well 4 may be formed a plurality ofassist gates A (An−2, An−1, . . . , An+2, and An+3) through a gate oxidefilm (tunnel insulator) having a thickness of about 7 nm. Those assistgates A may be used to control the potential of the surface of thesubstrate 1. The assist gates A may be made of, for example, an n-typepolycrystalline silicon film. In the upper part of each of the assistgates A may be formed a cap insulator 6 made of a silicon oxide film andin the upper part of the insulator 6 may be formed an interlayerinsulator 9 having a thickness of about 15 nm.

Word lines W (W0 to W66) that are also used as control electrodes may beformed in the upper part of the interlayer insulator 9. Those word linesW may be made of, for example, an n-type polycrystalline silicon filmand extended in a direction orthogonal to the extending direction of theassist gates A.

The width of each word line W may be, for example, 0.1 μm, and each wordline may be separated from its adjacent word lines by a side wall spacer12 made of a silicon oxide film having a thickness of about 20 nm. Inother words, in any of the conventional flash memories, the spacebetween word lines W may be about the width (gate length) of the wordline W itself. In the flash memory in this exemplary embodiment,however, the space between word lines W may be ½ of the width (gatelength) of the word line W and under.

At the bottom of the interlayer insulator 9 that separates the assistgates A from the word lines W may be provided charge storage regionsformed by silicon microcrystal grains 8 having an average diameter of 10nm. The charge storage regions may be diffused densely without cominginto contact with each another. Electrons may be injected into thosemicrocrystal grains 8 to store information in the flash memory.

Referring to FIG. 1, a memory array may be basically structured so as todispose, for example, 67 word lines W (W0 to W66) in the Y direction.(Hereinafter, such a memory array will be referred to as a memory mat.)The 64 word lines (W1 to W64) of those word lines W may be effective andthe rest word lines W0, W65, and W66 located at both ends of the Ydirection of the memory mat may be dummy word lines that do not functionas actual word lines W. Generally, the word lines located at both endsof a memory mat may be shifted in size significantly in a treatingprocess, and thus the writing characteristics to be varied among memorycells in the memory mat may be reduced by avoiding the use of those wordlines as memory cells. The total number of word lines per memory matincluding the dummy ones may be an odd number to allow for treating, aswill be described herein below.

Four adjacent assist gates A (for example, An−2, An+1, An, and An+1) maybe collected into a group, and such groups may be disposed repetitivelyin the X direction in FIG. 1. Also, an independent voltage may beapplied to each group of assist gates through control lines 22, 23, 24,and 25 extended in parallel to the word lines W. This means that thesame voltage may be applied to the assist gates (ex., A4, A8, A12, A16,. . . ), among which the remainder obtained by dividing n by 4 may beequal. The number of assist gates A may be, for example, 16904 (A0 toA16903) disposed in a 2048-byte region that includes a 512-bytemanagement region and four dummys disposed at each end of the Ydirection.

On the substrate 1 at both ends of the Y direction of the memory mat maybe formed a plurality of active regions S ( . . . , Sn−2, Sn−1, Sn,Sn+1, Sn+2, Sn+3, . . . ) with an isolation region between them,respectively.

A memory cell array may include 512 memory mats configured as describedabove, for example, in the Y direction.

The flash memory in this embodiment may use n-type inversion layersformed on the surface of the p-type well 4 when a positive voltage isapplied to the object assist gate A as a local data line D (see FIG. 5).Generally, this type of inversion layer may have a resistance higherthan that of the diffusion layer formed by means of high densityimpurity injection, so that the voltage to be applied to memory cellsmay differ among places of those memory cells in the memory mat duringan operation. The writing characteristic thus comes to be easily variedamong memory cells.

However, the flash memory in this exemplary embodiment may enable eachlocal data line D to be effectively reduced in length when the number ofword lines may be the same as that of any of the conventional flashmemories formed on the same design rules, since the space between wordlines W may be reduced to ½ of the width (gate length) of the word lineW and under. This is why the flash memory in this exemplary embodimentmay reduce the variation of the writing characteristic among memorycells, which may depend on where the object memory cell is positioned inthe memory mat.

An n-type diffusion layer (not shown) connected to assist gates Athrough contact holes 31 may be formed in the p-type well 4 in each ofthe active regions S ( . . . , Sn−2, Sn−1, Sn, Sn+1, Sn+2, Sn+3, . . . )formed at both ends of the Y direction of a memory mat. This n-typediffusion layer may be connected electrically to local data lines D whenn-type inversion layers (local data lines D) may be formed in the p-typewell 4 located in the lower part of each assist gate A. Also, aselection MISFET (Q) may be formed by the above described n-typediffusion layer and gate electrodes 21 and 26.

A global data line G (see FIG. 5) may be connected to each of the abovedescribed local data lines D through a selection MISFET (Q). The globaldata line G may be extended over a plurality of memory mats, and aplurality of local data lines D may be connected to one global data lineG in a hierarchical data line structure. Consequently, the data line mayhave less resistance when each of the local data lines D made of ainversion layer having a high resistance are extended, thereby reducingthe variation of the writing characteristic among memory cells, whichdepends on the place of each memory cell in the object memory mat. Inaddition, because no high data line voltage may be applied to any memorycell except when writing in a selected memory mat, the disturbance ofeach non-selected memory cell may be reduced. Also, because thecharging/discharging capacity may be reduced, the flash memory in thisexemplary embodiment may obtain affects of both fast operation and lowerpower consumption.

FIG. 6 is a schematic top view of a major portion of contact regionsprovided at both ends of an X direction of a memory mat, and FIG. 7 is across sectional view of the semiconductor substrate along a G-H line(cross sectional direction of word lines W) shown in FIG. 6.

If the space between word lines W is reduced to ½ of the width (gatelength) of the word lines W like the flash memory in this exemplaryembodiment, the layout of the contacts may be changed betweenodd-numbered word lines W (W1, W3, W5, . . . W65) and even-numbered wordlines W (W0, W2, W4, . . . W66). Contact holes 33 to be connected toodd-numbered word lines W(W1, W3, W5, . . . W65) may be disposed on wideportions (herein termed “dog bone parts”) formed at one end of thoseodd-numbered word lines W. The other ends that are opposite to the wideportions may be cut shorter and the contact holes 32 of theeven-numbered word lines W (W0, W2, W4, . . . W66) may be disposedaround those shorter ends. Each of the contact holes to be connected toany of the even-numbered word lines W (W0, W2, W4, . . . W66) may beprotruded partially into a region off the word line W. Because thecontact with each even-numbered word line may be assumed not only on thetop face, but also on a side face, the contact resistance may not bevaried so much even when the contact area on the top face is changedupon an alignment error in the lithography process. In addition, becausethe contact area increases such way, this structure may also beeffective for lowering the contact resistance.

The above described contact holes 32 and 33 may be disposed outsideactive regions S (isolation regions 2) of the object memory mat, so thatthe contact holes 32, even when some of them are disposed off the wordlines, may never be short-circuited with any other conductor layerelectrically. Also, odd-numbered word lines W that may be adjacent toeach other (ex., W1 and W3, W3 and W5, W5 and W7, . . . ) may bedisposed so that locations of their dog bone parts may become oppositeto each other, but not in contact with each other.

Next, a description will be made for the operation of the flash memoryin the above mentioned exemplary embodiment, with reference to FIGS. 5and 8, including a memory cell (enclosed in a circle in FIG. 5) drivenby a word line (W4), an assist gate (An), and an assist gate (An+1) withrespect to writing, erasing, and reading operations, for example.However, the operations may also be the same in any other memory cell inthe same memory mat; only the selected word line and assist gates may bechanged. In FIG. 5, the assist gates (An and An+1) located at both sidesof the object memory cell for writing may be omitted. The local datalines (Dn and Dn+1) made of inversion layers formed under the assistgates may be denoted (An and An+1) may be just denoted. Each chargestorage region consisting of a plurality of silicon microcrystal grainsmay be represented by a single white circle.

The flash memory in this exemplary embodiment may use 4 level thresholdvalues for each charge storage region consisting of microcrystal grains8 formed between assist gates (An and An+1) to store 2-bit data therein.At that time, the assist gates (An−1 and An+2) adjacent to the assistgates (An and An+1) may function as isolation regions. Also, four assistgates A may be connected into one set. Therefore, when information maybe written/read in from a memory cell located between the assist gates(An and An+1), the information may also be written/read in from thememory cell located between the assist gates having numbers differentfrom those of the assist gates (An and An+1) only by a multiple of 4,like, for example, the assist gates (An+4 and An+5).

The correspondence between each threshold value and information is shownin FIG. 9. It is represented as, for example,V3>V2H>V2L>V1H>V1L>V0H>V0L. The correspondence between 2-bit informationof “0”/“1” and a threshold level may be varied. The order of the writingthreshold levels may also be decided. Writing may be done in adescending order of the threshold levels. The voltage for one (An) ofthe assist gates (An and An+1) close to the charge storage region of thetarget memory cell used for writing may be set at 2 V, for example,which may be enough to form a inversion layer, and the other assist gate(An+1) may be set at, for example, 7 V. The voltage for the assist gatesadjacent to the assist gates (An and An+1) may be set at a low voltage,such as 0 V, for example, which cannot form any inversion layer toisolate them electrically.

Both the n-type diffusion layer and the local data lines (Dn and Dn+1)may be electrically connected to each other when the inversion layersare formed and a voltage is applied from the global data lines (Gn andGn+1) through the contact holes 31 connected to the diffusion layer.Those global data lines (Gn and Gn+1) may be set at a predeterminedvoltage with respect to selecting a control line 22 of the local dataline selection MISFET(Q). If the information to be written is not “01”,both local lines may be set at Vsw (0 V, for example). If theinformation to be written is “01”, the local data line (Dn) may be setat Vsw (0 V, for example) and the local data line (Dn+1) may be set at apredetermined voltage Vdw (4 V, for example), respectively.

If a writing pulse is applied to a word line (W4) that is a controlelectrode for a fixed time (3 μs, for example) at a high voltage Vww3(14 V, for example), a inversion layer may be formed in the p-type welllocated in the lower part of the word line (W4), electrical fieldconcentration occurs at the boundary with the local data line (Dn)located in the lower part of the assist gate (An), thereby hot electronsmay be generated. The generated hot electrons may be pulled into theelectrical field vertical to the substrate 1 through the word line (W4)and may be injected into the target memory cell. At that time, becausethe resistance of the local data line (Dn) located in the lower part ofone assist gate (An) may be high, the current that flows between thelocal data lines (Dn and Dn+1) may not be so large. Consequently, thecurrent may not increase much even when information is written in manymemory cells simultaneously. This is why information may be written inmany memory cells in parallel, even at a current driving performance ofa limited step-up circuit, and why the flash memory may be considered tobe suitable for files from which information consisting of many bits isinputted/outputted at a time. Such hot electron injection may bereferred to as source side injection.

If the information to be written is not “01”, no potential differencemay occur between the local data lines (Dn and Dn+1). At that time,therefore, no hot electron may be generated and accordingly no chargeinjection may occur. Also, if the channel of a memory cell to be drivenby a non-selected word line W is disconnected electrically from anotherwhen a not-selected word line W is set at a low voltage (for example, 0V), no information may be written in the memory cell.

A fixed high potential Vdw may be set for one (Dn) of the local datalines (Dn and Dn+1) when writing. However, it may also be possible toemploy another driving method in which a high potential may be used to:pre-charge the local data line (Dn); turn off a switch provided betweenthe local data line (Dn) and a power supply line to set the line (Dn) inthe floating state; and apply a writing pulse to the object word line W.If a fixed voltage is used for driving an object memory cell, thewriting current may be varied due to the high resistance of the localdata line made of an inversion layer. However, the pre-charging methodmay fix the charge, and the variation of the writing characteristicamong memory cells may be reduced. This may also be true in otherexemplary embodiments described later.

In the configuration of the flash memory of an exemplary embodiment, ifinjected electrons diffuse in a direction orthogonal to a word line,information may be written in an adjacent memory cell, since the wordline may be close to its adjacent word lines. The source side injectionmay resolve such a circumstance, since the region in which hot electronsmay be generated may be narrower than that of the drain side injection,and the energy of generated hot electrons may also be distributed inuniform, thereby generated electrons may not diffuse so much in thedirection (parallel to the assist gates) orthogonal to any word line W.

In the above described writing operation in this embodiment, if thepotential set for the source side assist gate (An) is further raised (to3 V, for example) and the resistance of the local data line (Dn) islowered, the potential difference between a word line W and the drainside local data line (Dn+1) may become smaller than that between theword line W and the source side local data line (Dn). Thus, theelectrical field around the drain may become stronger than that aroundthe source. Hot electrons thus come to be generated around the drain andthe charge storage region becomes closer to the drain. This drain sideinjection may also be employed, of course. Also, because the drain sideinjection lowers the resistance of the local data line (Dn), the localdata line voltage drop may be suppressed, thereby the variation of thewriting characteristic among memory cells, which depends on where eachtarget memory cell may be located in the memory mat, may be reduced.

The charge storage nodes provided at both sides between the adjacentelectrodes (An and An+1) may also be used to increase the storagecapacity. Accurate controlling of the charge injection amount requiredin the above storing method using 4-level charge injection amounts maynot be required. The verification operation may thus be simplified, andthereby the writing speed may be improved. In addition, because thedifference between the minimum threshold level and the maximum thresholdlevel may be reduced, the writing voltage may be lowered and writteninformation may be more stably retained. In order to realize suchstoring of information, the source side injection and the drain sideinjection as described above may be combined and the set voltages ofboth source side and drain side assist gates (An and An+1), as well asthose of the local data lines (Dn and Dn+1), may be exchanged when inthe source side injection.

After that, a reading operation may be done from the memory cell toverify whether or not the threshold value Vth may be higher than V3. Thedetails of the reading operation will be described below. If theinformation to be written is “01” and the threshold value Vth is nothigher than V3, the local data line (Dn+1) may be set at thepredetermined voltage Vdw (4 V, for example) again. If the thresholdvalue Vth is higher than V3, the local data line (Dn+1) may be set atVsw (0 V, for example), and then the writing pulse may be applied to theword line (W4). After that, the information may be read again to verifyit and the writing pulse may be applied to the word line (W4) again, ifnecessary. Such an operation sequence may be repeated until targetinformation may be read completely.

In the memory cell array configured in this exemplary embodiment,adjacent memory cells may be used for electrical isolation. Therefore, awriting operation may be done for the assist gates A of one of the fourmemory cells selected from among those driven by the same word line(W4). When all the memory cells subjected to the writing pass theverification test, the “01” writing sequence may end. After that, thesystem may go to the “00” writing sequence.

In such a case, if the information to be written is “00”, the voltage ofthe local data line (Dn+1) may be set at the predetermined voltage Vdw(4 V, for example). If not, the voltage of the local data line (Dn+1)may be set at the same voltage Vsw (0 V, for example) as that of thelocal data line (Dn). The potential set for the assist gates A may bethe same as that of the “01” writing sequence. After that, the writingpulse may be applied to the word line (W4). The voltage Vww2 of thewriting pulse may be lower than the Vww3, at, for example, 12 V. At thattime, if the pulse width is the same as that of the “01” writing, thecharge electrons to be injected may be less and the writing may be doneon a lower threshold level. The verification test may also be done inthe same way as that described above, except that the threshold valuemay need to be set higher than V2L and lower than V2H. It may berequired that charge electrons may not be injected excessively with thefirst writing pulse, and the width of the second and subsequent pulsesmay be reduced, to prevent excessive injection of charge electrons. Whenall the memory cells in which information is written pass theverification test, the “00” writing sequence may end, and then thesystem may go to the “10” writing sequence. In the “10” writing, thewriting voltage Vww1 may be set lower than Vww2, at, for example, 10 V,and the target threshold range may be changed from that of the “00”writing. Other operations may be the same as those of the “00” writing.After that, the “01” writing sequence may be done and the writing in thememory cell may be completed.

Although the writing pulse voltage to be applied to the word line (W4)through each sequence may be fixed when writing information, it may alsobe possible to use a pulse string that applies a higher voltageaccording to an increase of the writing frequency to end the objectwiring sequence faster.

Erasing information from a plurality of memory cells driven by the sameword line may be done. A positive voltage Vew (ex., 20 V) higher thanthe Vww3 may be applied to the word line W. The potential of the chargestorage region may be low after electrons have been injected therein.The electrical field of the inter-layer insulator 9 thus may becomestronger than that of the tunnel insulator (gate oxide film 5). As aresult, electrons may be ejected toward the control electrode (W4), andthereby the threshold value of the memory cell may go lower. The erasingmethod is not limited to only this example, and may be changedaccordingly. For example, a negative voltage (ex., −18 V) may be appliedto the target word line W and electrons may be ejected toward thesubstrate 1. It may also be possible to apply a negative voltage (ex.,−3 V) to the p-type well 4 and a positive voltage (ex., 3 V) to thelocal data lines (Dn−2, Dn−1, Dn, Dn+1, Dn+2, and Dn+3), then a negativevoltage (ex., −13 V) to the word line, to inject holes therein to eraseinformation therefrom. This hole injection may enable information to beerased only from selected memory cells by selecting the inversion layersto be set at a negative voltage.

To read 4-level information, at first it may be determined whether ornot the threshold level is “00” and over, that is, V2L and over, or “10”and under, or VH1 and under. The local data line (Dn) may be pre-chargedto a lower potential Vsr (ex., 0 V) and the local data line (Dn+1) maybe pre-charged to a higher potential Vdr (ex., 1.0 V) through the globaldata lines (Gn and Gn+1).

After that, a voltage Vrw1 of V1H<Vrw1<V2L may be applied to the wordline (W4). If the threshold value level of the object memory cell is V1Hand under, the local data line (Dn) and the local data line (Dn+1) maybe connected electrically to each other. If the threshold value level isV2L and over, they may not be connected electrically to each other, orthey are high in resistance. If the result is V1H and under, the localdata lines (Dn) and (Dn+1) may be pre-charged as described above, andthen a voltage Vrw0 of V0H<Vrw0<V1L may be applied to the word line (W4)to determine whether the threshold value level of the object memory cellis “1” or “10”, according to the difference between the currents flowingin the word line (W4). If the first reading result is V2L and over, thelocal data lines (Dn) and (Dn+1) may be pre-charged as described above,and then a voltage Vrw2 of V2H<Vrw2<V3 may be applied to the word line(W4). After that, the threshold value level may be “00”, “10”, or “01”,according to the difference between the currents flowing in the wordline (W4).

In the above reading operation, it may also be possible to read objectinformation after completing the reading operations with use of Vrw0,Vrw1, and Vrw2 without changing the voltage to be applied, according tothe result obtained with use of Vrw1. The former reading method may besuitable for fast reading, since it may require voltage application justtwice while the latter method may simplify the control circuit, althoughit may require voltage application three times.

If the charge storage regions at both sides as described above are used,the local data lines (D) and (Dn+1) may be changed in function to readobject information. The pre-charge voltage Dvr for reading may be sethigher than that of the maximum threshold value level. As a result, ifthe voltage Vdr is applied to the local data line (Dn+1), a channelpinch-off event may occur around the local data line (Dn+1), so thatinformation may be read from the charge storage region around the localdata line (Dn) without being affected by the information stored in thecharge storage region around the local data line (Dn+1). Similarly, if avoltage Vdr is applied to the local data line (Dn), information may beread from the charge storage region around the local data line (Dn+1).

In this exemplary embodiment, the even-numbered word lines (W0, W2, W4,. . . , W66) and the odd-numbered word lines (W1, W3, W5, . . . , W65)may be formed in different processes. This characteristic may differbetween adjacent word lines. Because the word line width, the interlayerinsulator thickness, etc. may be different between the even-numberedword lines and the odd-numbered word lines, the writing, erasing, andreading characteristics might differ between even-numbered word linesand odd-numbered word lines. As shown in FIG. 10, a solution to this maybe that the voltage generated by the regulator provided in the voltagegenerator may be changed according to whether the object word line iseven numbered or odd-numbered so as to enable the operation voltage tobe changed.

Although a word line voltage may be used to compensate thecharacteristic difference between even numbered word lines andodd-numbered word lines in this exemplary embodiment, a pulse width maybe used for the same purpose. In addition, the data line voltage and thevoltage to be applied to assist gates may be changed according towhether the object word line is even numbered or odd numbered. Suchmethods may also be used in other preferred embodiments of the presentinvention as described herein to obtain the same effect.

It may also be possible to control the voltage of each assist gateaccording to the position of the target memory cell in the memory mat tosuppress the variation of the writing characteristic among memory cells,which may depend on the position of each target memory cell in thememory mat. For example, the circuit configuration as shown in FIG. 11may be employed to change the supply voltage according to how far theaddress of a target word line W selected when writing may be separatedfrom the contact with the corresponding high voltage side local dataline in the memory mat. If the contact is close in position, it may meanthat the address may be far from the low voltage side contact.Consequently, if a writing current flows due to a voltage drop and thecontact is close in position, both the source and drain voltages of thesubject memory cell may rise higher than when the contact is far inposition. The current thus decreases and the word line voltage may golower than that of the source region, which may be a reference voltage.Accordingly, the writing may often slow down.

A higher voltage may be set for the assist gate A corresponding to thelow voltage side local data line. The source side voltage may thus besuppressed from increasing, and thereby the characteristics of bothsides may become identical. In the case of the assist gate controllingas described above, the voltage may be changed for each address insteps. However, it may also be possible to divide a plurality of wordlines W into groups and apply different voltages to those groups. Thismethod may simplify the assist gate controlling more.

Referring to FIGS. 12 through 21, producing the flash memory in anexemplary embodiment is shown. Here, a description is made for a memorycell array.

At first, an isolation region 2 and an active region S may be formed onthe substrate 1 with use of a known isolation technique. FIG. 12 shows atop view of the active region S and its peripheral isolation region 2formed in a memory mat. As shown in FIG. 12, the isolation region 2 maybe formed only at each assist gate bundling part provided at an end ofthe memory mat, at the contact lead-out part of each inversion layer(local data line), and at each word line contact part.

Next, as shown in FIG. 13, the substrate 1 may be subjected to animpurity ion injection process to form an n-type well 3 and a p-typewell 4, and then to an ion injection process so as to adjust thethreshold voltage. After that, the substrate 1 may be subjected to athermal treatment process to form a gate oxide film (tunnel insulator) 5at a thickness of about 7 nm on the surface of the p-type well 4.

After that, assist gates A may be formed on the gate oxide film 5 bydepositing an n-type polycrystalline silicon film with use of the CVDmethod, then depositing a cap insulator 6 made of oxide silicon thereonwith use of the CVD method. After that, the cap insulator 6 and then-type polycrystalline film may be patterned with use of a photographytechnique. FIG. 14 shows a top view of some assist gates A (An−2, An−1,An, An+1, An+2, and An+3). The actual number of the assist gates A maybe, for example, 16904 formed in a 2048-byte region that includes a512-byte management region and 8 dummy assist gates.

After that, the substrate 1 may be subjected to another ion injection toadjust the impurity density on its surface, then silicon microcrystalgrains 8 of about 10 nm in diameter may be formed on the substrate 1 ata density of 4×10¹¹/cm². At that time, but prior to the forming of themicrocrystal grains 8, the gate oxide film 5 formed in the space regionof each assist gate A may be removed by hydrofluoric acid to expose thesurface of the p-type well 4. Then, the substrate 1 may be subjected toa thermal treatment process to form a clean silicon oxide film of about8 nm in thickness on the surface of the p-type well 4.

After forming the microcrystal grains 8, the surface may be oxidized byplasma oxidation up to a depth of 4.5 nm, and then may form anotherlayer of microcrystal grains 8 in the upper part of the preceding layerof the microcrystal grains 8. Although this method may form themicrocrystal grains 8 at a higher density, those grains 8 may not comein contact with each another. Consequently, more electrons may betrapped than by other methods under the same writing condition. This maymake it possible to secure more margin between written informationitems, and thereby the writing characteristic may be stabilized.

Plasma oxidation and forming of microcrystal grains 8 as described abovemay be repeated several times to form the microcrystal grains 8 in aplurality of layers. Unlike the thermal oxidation, plasma oxidation mayoxidize the object within the distance of the radical invasion. This iswhy plasma oxidation may prevent the microcrystal grains 8 formed in thefirst step from becoming very small in size, even when the plasmaoxidation process is repeated by several times and the tunnel insulatorfilm (gate oxide film 5) grows excessively in deposition.

As shown in FIG. 16, an interlayer insulator 9 may be deposited on thesubstrate 1, and then a polycrystalline silicon film 10 may be depositedon the insulator 9 with use of the CVD method. After that, the surfaceof the silicon film 10 may be flattened by the CMP (Chemical MechanicalPolishing) method. The inter-layer insulator 9 may be formed as athree-layer one consisting of an silicon oxide film of about 5 nm, asilicon nitride film of about 8 nm, and a silicon oxide film of about 5nm in thickness.

A silicon nitride film 11 of about 25 nm in thickness may then bedeposited in the upper part of the silicon film 10 by the CVD method.After that, as shown in FIG. 17, the silicon nitride film 11 and thepolycrystalline silicon film 10 may be patterned with use of thephotography technique to form word lines W, each being of apolycrystalline silicon film 10 covered by the silicon nitride film 11.FIG. 18 shows a top view of some of the word lines W. As shown in FIG.18, word lines W formed in that process may be even-numbered (W0, W2,W4, . . . , W66) among the 67 word lines (W0, W1, W2, . . . , W66)described above. At that time, the gate electrodes 21 and 26 of theselection MISFET (Q) may also be formed.

Also, as shown in FIG. 19, the substrate 1 may be subjected to a thermaloxidation process to form a side wall spacer 12 of a silicon oxide filmof about 20 nm in thickness at a side face of each word line W(polycrystalline silicon film 10) that might not be covered by thesilicon nitride film 11. At that time, the silicon nitride film formedin the interlayer insulator 9 that covers both assist gates A andmicrocrystal grains 8 may have resistance to oxidation, so that none ofthe assist gates A and the microcrystal grains 8 may be oxidized.

After that, as shown in FIG. 20 and FIG. 21, word lines W (W0, W2, W4, .. . , W66), each consisting of an n-type polycrystalline silicon film,may be formed in their space regions. The word lines W formed in thatprocess may be odd-numbered (W1, W3, W5, . . . , W65) among theabove-described 67 word lines (W0 to W66).

The word lines W described above may be formed by depositing an n-typepolycrystalline silicon film on the substrate 1 with use of the CVDmethod, then flattening the surface of the silicon film with use of theCMP method. When the CMP method is used to polish the polycrystallinesilicon film, the polishing may stop on the surface of the siliconnitride film 11 that covers the top face of each of the word lines (W0to W66) formed as described above. As a result, a level differenceequivalent to the thickness of the silicon nitride film 11 may appearbetween the top faces of the even numbered word lines (W0, W2, W4, . . ., W66) formed first and the top faces of the odd-numbered word lines(W1, W3, W5, . . . , W65) formed after them.

After that, an interlayer insulator (not shown) may be formed in theupper part of each word line W, and then contact holes 31 to 33 shown inFIG. 1 may be formed. After that, control lines 22 to 25 of the assistgates A may be formed by the metallic line in the first layer. Then, aninterlayer insulator (not shown) may be formed in the upper part of thecontrol lines 22 to 25, then global data lines G (see FIG. 5) may beformed by the metallic line in the second layer.

In this exemplary embodiment, wells may all be formed as p-type, andelectrons may be used as carriers. However, the wells may be n-type onesand holes may be used as carriers. In the latter case, the strong-weakrelationship between voltages may be reversed from that illustrated inthis embodiment. This may also be true in other embodiments of thepresent invention.

In addition, although the cap insulator 6 that covers the top faces ofthe even numbered word lines (W0, W2, W4, . . . W66) formed may beformed as a silicon nitride film, any other insulator material may beused for the insulator 6 if it has an etching selection ratio adequatewith respect to the conductor material used for forming the word lines.For example, a silicon oxide film may do. The material for forming wordlines W may also become different between even numbered word lines (W0,W2, W4, . . . , W66) and odd-numbered word lines (W1, W3, W5, . . . ,W65).

The microcrystal grains 8 used to form the charge storage regions may beformed by a semiconductor material other than silicon or metallicmaterial. They may also be formed by an insulation material (such assilicon nitride film, for example) having a charge trapping function. Ifthe charge storage regions are formed by microcrystal grains 8 as inthis exemplary embodiment, memory nodes may be insulated from eachother. Therefore, there may be no need to isolate those memory nodesfrom each other, although it may be required for the memory nodes ineach conventional flash memory. This is why such memory nodes may betreated similarly to this embodiment. The same effect may be obtainedeven when an insulation material having a charge trapping function isused for forming the charge storage regions.

If microcrystal grains 8 are to be used to form charge-storage regions,the periphery of each charge-storage region may be enclosed by such aninsulation material as a silicon oxide film having no trapping function.Therefore, it may be possible to select a material that does not causecharge transfer between microcrystal grains 8 easily. Consequently,charge storage regions may come to have excellent charge retainingcharacteristics. This method may thus be suitable for multi-bit memorytype flash memories with less threshold variation margin. If word linesare disposed very closely with each other as in this exemplaryembodiment, and charge transfer may occur in a direction orthogonal tothe extended direction of the word lines W, a specific problem mayarise. The characteristics of adjacent memory cells may become varied.This is why it is recommended to use microcrystal grains 8 to formcharge storage regions.

Because the silicon nitride film may enable etching to have a selectionratio with respect to the silicon oxide film, it may be easier to treatthe silicon nitride film than the microcrystal grains 8. Although theinterlayer insulator 9 that isolates charge storage regions(microcrystal grains 8) from gate electrodes W may be formed as alaminated film of a silicon oxide film, a silicon nitride film, and asilicon oxide film in this exemplary embodiment, the insulator 9 mayalso be formed of alumina. Alumina has resistance to oxidation, so itmay have the same effect as that of the silicon nitride film. Inaddition, because alumina is high in dielectric constant and the voltageapplied to each word line W may be transferred efficiently between eachcharge storage region and the surface of the substrate 1, writing may bedone fast. The description for the configurations of the charge storageregion and the interlayer film may also apply to other embodiments ofthe present invention.

FIGS. 22-24 are further illustrative of an exemplary embodiment of thepresent invention. FIG. 22 is a cross sectional view of a major portionof a semiconductor substrate of a flash memory. FIG. 23 is a circuitdiagram of an equivalent circuit during a writing operation, and FIG. 24is a schematic top view of a major portion of contact regions providedat both ends of an X direction of a memory mat.

The flash memory in this exemplary embodiment differs in the method ofwriting operation. As previously described, writing may be done betweentwo adjacent assist gates (An and An+1) and isolation may be done at theassist gates (An−1 and An−2) next to those electrodes (An and An+1). Inthis exemplary embodiment, however, writing may be done among anadjacent three assist gates (An−1, An, and An+1).

A positive voltage may be applied to each of the two assist gates (An−1and An+1) of the three assist gates, which may be located at both ends(ex., 3.5 V to (An−1) and 7 V to (An+1) of the memory mat, to forminversion layers (Dn−1 and Dn+1) under those electrodes. These inversionlayers (Dn−1) and Dn+1) may receive their power from a diffusion layerprovided at an end of the memory mat. In this case, there is no need toset one (Dn−1) of the inversion layers at a high resistance.

If a high voltage Vww (15 V, for example) is applied to a word line (W4)as a writing pulse, the resistance may go low at every place except forthe surface of the substrate 1 under the corresponding assist gate. If apotential difference exists between diffusion layers, an electricalfield may be concentrated under the right end portion of the assist gate(An), thereby hot electrons may be generated there. Those hot electronsmay be pulled toward the word line (W4), since the potential of the wordline (W4) may be high, and may go over the potential barrier wall of thetunnel insulator (gate oxide film 5) to be injected in the chargestorage region (enclosed by a circle (a) in FIG. 22). When writinginformation in the left side charge storage region (enclosed by a circle(b) in FIG. 22) of the center assist gate (An), it may be required toreplace the set voltage of the assist gates at both sides (An−1 andAn+1) with the set voltage of the inversion layers (Dn−1 and Dn+1) atboth ends. Writing may also be possible by using any of the assist gates(An−1 and An+1) used as the assist gates at both ends in the abovewriting operation as a center assist gate. If the assist gate (An−1) isused as a center one, writing may be done in its right side region(enclosed by a circle (c) in FIG. 22). If the assist gate (An+1) is usedas a center one, writing may be done in its left side region (enclosedby a circle (d) in FIG. 22). As a result, information may be stored intwo places between adjacent assist gates (ex., An and An+1, as well asAn−1 and An) as described previously.

In such a writing operation, a low potential inversion layer (Dn in theearlier exemplary embodiment and Dn−1 in the current embodiment) may beset at a high resistance, and an electrical field may be concentrated atan end portion. Therefore, the resistance value of the inversion layer(local data line D) may change significantly depending on the positionof the subject word line W in the memory mat. In this exemplaryembodiment, therefore, the center assist gate may form a high resistanceregion, so that such a position dependency may be eased. In other words,this exemplary embodiment may effectively reduce the writing variationamong memory cells more.

In this exemplary embodiment, erasing may be done as previouslydescribed. Also, reading may be done from between inversion layersformed by adjacent assist gates differently from the writing done inunits of three assist gates. The equivalent circuit used at that timemay be that shown in FIG. 5. The sensing method may be the same as thatdescribed earlier. In this exemplary embodiment, dummy memory cells maybe prepared and three level threshold values may be set in those dummymemory cells. Sensing in the reading method may be done as adifferential operation from the target memory cell. The threshold valuemay be set between adjacent levels of the four level threshold valuesdenoting the memory cell states. The method makes it possible to reducethe time between applying a reading pulse to a target word line W andstarting of a sense amplifier. The reading speed may thus be increased.While the method employed in an earlier embodiment may be effective toeliminate the apprehension of the variation of writing characteristicamong the dummy memory cells, the reliability of the flash memory in theearlier embodiment may be improved.

Also, as shown in FIG. 25, the dummy memory cells may be divided intothose for even numbered word lines and those for odd-numbered wordlines. The word lines (WDE0, WDE1, and WDE2) of the dummy memory cellsfor even numbered word lines may be formed concurrently with the evennumbered word lines, and both laminated structure and contactdisposition may be the same among them. The word lines (WDO0, WDO1, andWDO2) of the dummy memory cells for odd-numbered word lines are alsoformed concurrently with the odd-numbered word lines, so that bothlaminated structure and contact disposition may be the same among them.Consequently, the flash memory in this second embodiment may compensatecharacteristic differences to be caused by processes.

A sense amplifier that makes a differential operation between two datalines (Dk) and (Dk+4) with three data lines therebetween may be used inthis embodiment. This may be because the assist gates may operate inunits of 4. Therefore, data line numbers separated from each another byan integer multiple of the number of data lines assumed as controlledmay preferably be paired to make it easier to control them. If assistgates may be controlled in units of two just like in other embodimentsdescribed herein, data line numbers separated from each other by aninteger multiple of 2 may be paired.

In this embodiment, the layout and manufacturing method of contacts maybe different from those in the first embodiment. As shown in FIG. 24,odd numbered word lines may be short and contact holes 32 may be usedfor even numbered word lines. These may be provided so as to avoidadjacent word lines so that no short-circuit may occur between anycontact hole 32 and its adjacent odd numbered word lines, even uponalignment error occurrence. Odd numbered word lines may be formedshorter than even numbered word lines at their one ends, and longer atthe other ends. In this embodiment, however, odd numbered word lines maybe all shorter than even numbered word lines at both ends.

Also, around the contact hole 33 of each odd-numbered word line mayexist its adjacent even numbered word lines. Short-circuiting may thusbe prevented between them, even at alignment error occurrence, if alarge selection ratio is assumed between the silicon nitride film andthe silicon oxide film in the etching process for forming the contactholes 33. This may be because even numbered word lines may be protectedby the cap insulator 6 consisting of a silicon nitride film. Such ashort-circuiting may also be prevented by limiting the over-etching onlyup to the thickness of the cap insulator and under. In that connection,contact holes 32 of even numbered word lines and contact holes 33 of oddnumbered word lines may be formed in different processes.

Some of the contact holes 32 of even numbered word lines, as describedin the exemplary embodiments, may be shifted from their word lines so asto cover space regions. (This method may be referred to as the side facecontact forming method.) As described above, if a large selection ratiois assumed between the silicon nitride film and the silicon oxide filmin the etching process for forming contact holes, the top face of eacheven numbered word line may be covered by the silicon nitride film (capinsulator 6), so that they are not connected electrically with eachother even when holes may have been formed from above. If the side facecontact forming method as described above is employed, contact holes ofboth even numbered and odd numbered word lines may be formedconcurrently.

The contact forming method in other embodiments may be employed, or anyof the contact forming methods to be described in the subsequentembodiments may be employed. The contact forming method in the earlierembodiments require no silicon nitride film (cap insulator 6) whenforming contact holes. After processing word lines, therefore, the capinsulator 6 may be removed and the top face of each word line may besubject to a silicidation process (to lower the resistance) so as tospeed up the operation. On the other hand, the contact hole formingmethod in this embodiment may require no wide portion (dog bone part) atthe places where contact holes 33 may be formed for odd numbered wordlines, so that no fine mask pattern may be required in the process forforming the odd-numbered word lines. Consequently, the process marginmay be secured more and the manufacturing cost of photo masks may bereduced.

As can been seen in FIGS. 26-29, yet another exemplary embodiment isillustrated. FIG. 26 shows a cross sectional view of a major portion ofa semiconductor substrate of the flash memory. FIG. 27 shows a schematictop view of a memory mat. FIG. 28 is a circuit diagram of an equivalentcircuit during a writing operation. FIG. 29 is a circuit diagram of theequivalent circuit during a reading operation.

In other embodiments of the present invention, a voltage may be appliedto each assist gate A to form a inversion layer to be used as a localdata line. In this embodiment, however, impurity ions may be injected ata high density in each p-type well 4 to form a diffusion layer 7 to beused as a local data line.

Writing in this embodiment may be similar to that in other embodimentsdescribed herein. In the case of writing, it may be required to make thelocal data lines (Dn−1 and Dn+1) in an earlier embodiment correspond tothe local data lines (Dn and Dn+1) in this exemplary embodiment. Inother words, just like writing of information in the right side region(a) of the assist gate An, information may be written in the right sideregion (e) of the assist gate An, and just like writing of informationin the left side region (b) of the assist gate An, information may bewritten in the left side region (f) of the assist gate An in thisexemplary embodiment. In the structure of the flash memory, theresistance of each local data line D when in writing may be lower thanpreviously described, thereby reducing the variation of writingcharacteristic among memory cells, which may depend on the place of eachtarget memory cell in the memory mat.

On the other hand, when in reading, a voltage may be applied to a targetassist gate (An) to form a inversion layer 14. This inversion layer 14may be set at a fixed voltage Vs (0 V, for example) through the sourceline 15. As a result, when in reading, information may be read from bothright and left sides of the assist gate independently. Also, becauseword lines W may be disposed at narrow pitches, the resistance of theinversion layer 14 may be reduced when in reading, thereby reducing thevariation of reading characteristic among memory cells, which may dependon the position of each subject memory cell in the memory mat just likein the first and second embodiments. However, while each inversion layer(local data line) may be used in both writing and reading in the firstand second embodiments, the inversion layer (local data line) may beused only for reading in this exemplary embodiment.

As shown in FIG. 27, in the configuration of the flash memory, only onetype voltage Vs may be required to be supplied to each inversion layerformed by a control electrode (word line W). The voltage Vs may becommon for all the assist gates. However, source lines 14 and 15 may bedisposed at the upper and lower ends of a memory mat. Consequently, theinversion layer manufacturing may be easier than in previously describedembodiments in which a inversion layer leading-out structure may berequired to be formed for each assist gate A. In addition, because theremay be no special operation required for forming inversion layers whenin writing, assist gates A may be controlled easily and the controllines 27 and 28 for the assist gates A may be required as one at each ofthe top and bottom of a memory mat. The area of the memory mat may thusbe reduced. Yet another exemplary embodiment is illustrated in FIGS. 30and 31. FIG. 30 is a cross sectional view of a major portion of asemiconductor substrate of a flash memory (along the cross sectionaldirection of the assist gates). FIG. 31 is a cross sectional view of amajor portion of the semiconductor substrate just after word lines W areformed (along the cross sectional direction of the word lines).

The flash memory may be similar to that in other previously describedembodiments with respect to the array configuration and the operation.However, each charge storage region in the flash memory in thisembodiment may consist of a polycrystalline silicon film 40. This may bea difference from previously described embodiments.

If a continuous polycrystalline silicon film 40 is used to form a solidstructure, an area in which word lines W face charge storage regions maybe larger, so that a voltage applied to each word line W may also beapplied to the tunnel insulator (gate oxide film 5) between each chargestorage region and the substrate 1 effectively, thereby writing/erasingspeed may be increased and the reading current increased. In order torealize the structure of the flash memory, however, this embodiment mayrequire a manufacturing process that may not be provided in any of theother embodiments. Hereinafter, the manufacturing process provided forthis particular embodiment will be described briefly.

The processes up to the forming of assist gates A and the tunnelinsulator (gate oxide film 5) may be the same as those in otherembodiments. After that, an n-type doped polycrystalline silicon film 40of about 25 nm in thickness may be deposited, then may be coated by aphoto-resist film, then may be subjected to an etching-back process toremove the photo-resist film from the upper part of each assist gate A.Then, the polycrystalline silicon film 40 in the upper part of eachassist gate A may be removed by etching and the photo-resist film may beremoved to form each charge storage region consisting of apolycrystalline silicon film 40 on the side wall of each assist gate andin the space region.

Next, an interlayer insulator 9 may be formed. The insulator 9 mayconsist of a silicon oxide film of about 15 nm in thickness. After that,an n-type doped polycrystalline silicon film may be deposited on theinterlayer insulator 9 and the surface may be flattened by the CMPmethod, and then a silicon oxide film may be formed in the upper portionthereof. After that, both of the silicon oxide film and thepolycrystalline silicon film may be etched using the photo-resist filmas a mask to form even numbered word lines W (W0, W2, W4, . . . , W66).

Next, an insulator of about 20 nm may be formed at both top face andside face of each word line W in a thermal oxidation process, then ann-type doped polycrystalline silicon film may be deposited andflattened. After that, the polycrystalline silicon film on even numberedword lines W (W0, W2, W4, . . . , W66) may be removed to formodd-numbered word lines W (W1, W3, W5, . . . , W65). Then, the siliconoxide film may be etched by anisotropic dry-etching. If a slower etchingrate of the polycrystalline silicon is selected at that time, thesilicon oxide film that separates even numbered word lines W fromodd-numbered word lines W may be removed and the etching stops on thesurface of the polycrystalline silicon film 40 of the charge storageregion. After that, the polycrystalline silicon film 40 between evennumbered word lines W and odd-numbered word lines W may be etched toform charge storage regions with use of word lines W and theself-alignment action. The subsequent processes are the same as those inthe other embodiments.

FIG. 32 shows a cross sectional view of a major portion of asemiconductor substrate of a flash memory in yet another exemplaryembodiment (along the cross sectional direction of assist gates). Theflash memory may be characterized in that each charge storage region maybe configured by a continuous silicon nitride film 41 of about 30 nm inthickness. The silicon nitride film 41 may have a charge trappingfunction, so that it may be used as charge storage regions even when itmay be formed as an insulator. Instead of the silicon nitride film 41,an alumina film may be used as an insulator having a charge trappingfunction.

If such each charge storage region is configured as an insulator, it maykeep the non-volatile property even when the tunnel insulator/interlayerinsulator 9 is reduced in thickness. The tunnel insulator may beconfigured by, for example, a gate oxide film 5 of about 1.5 nm inthickness and the interlayer insulator 9 may be configured by a siliconoxide film of about 2 nm in thickness.

The flash memory may also be characterized by the locations of thecharge storage regions. In addition to the charge storage regionsenclosed by a circle (a) in FIG. 32, respectively, each of the chargestorage regions (enclosed by a circle (d) in FIG. 32) of its adjacentassist gate (An+1) may be used as a charge storage region of the assistgate (An). Also, while the flash memory in the first embodiment mayadjust the charge to be stored in one charge storage region to set 4level threshold values and store 2-bit information, the flash memory maystore 2-bit information in two charge storage regions. In this exemplaryembodiment, a 2-level threshold value may be set in each of the twocharge storage regions to store single-bit information.

This may be because a threshold value may be apt to be varied slightlyif such an insulator having a charge trapping function may be used asthe silicon nitride film 41 and an alumina film may be used for chargestorage regions, so that it may be recommended to store single bitinformation in one charge storage region. Also, if an insulator having acharge trapping function is used for charge storage regions, the chargetrapping may be done easily at a high density, so that a charge amountmay be secured even when charge electrons may be injected in a narrowregion. The flash memory operation may be thus stabilized, so that thismethod may be suitable for the flash memory as described.

Because the charge storage regions may be located differently from thosein other embodiments, some operations of the region may also differ fromthose previously described. When writing information in the chargestorage region (a) close to the assist gate (An), the writing may bedone similarly to that in other embodiments. When writing information inthe charge storage region (d) at the side of the assist gate (An+1),however, the functions of the two assist gates (An and An+1) may beexchanged. At that time, the global data line Gn may be replaced withanother Gn+1 for loading data.

Such a functional exchange may also be done for reading information. Thevoltage of the reading drain at that time, that is, the voltage Vd to beapplied to the inverted layer, may be set so as to satisfy arelationship of VWr−Vth1<Vd when the low threshold value for storinginformation may be Vthl in status and the reading voltage to be appliedto the object word line W may be VWr. Consequently, the information inthe drain side charge storage region used for a pinch-off event may beblocked at the drain side, so that only the source side information maybe read. For example, when reading information from a charge storageregion close to the assist gate (An), 0 V may be set as the voltage ofthe inverted layer (local data line Dn) formed by the assist gate (An)and 2 V may be set as the voltage of the inverted layer (local data lineDn+1) formed by the assist gate (An+1). When reading information fromthe charge storage region (d) close to the assist gate (An+1), 0 V maybe set as the voltage of the inverted layer (local data line Dn+1) and 2V may be set as the voltage of the inverted layer (local data line Dn),respectively.

Erasing may be done in this embodiment as previously described. However,it may also be possible to erase information by generating a pair ofelectron holes at the substrate side, then applying a negative voltageto the object word line W to inject holes therein. The hole injectionerasing method may be characterized in that it requires a comparativelylow voltage to be applied to erase information fast.

While the forms of the present invention has been described in detail,it is to be understood that modifications will be apparent to thoseskilled in the art without departing from the spirit of the invention.

According to the present invention, in a non-volatile semiconductormemory device that uses each inverted layer formed on the surface of thesemiconductor substrate as a data line, it may be possible to achieveboth a reduction of the variation of the writing/reading characteristicsamong memory cells and reduction of the bit cost.

1. A semiconductor memory device, comprising: at least one assist gateformed through a first insulator on a main surface of a first conductivetype semiconductor substrate and extended in a first direction of saidmain surface; a plurality of word lines formed on said assist gatesthrough a second insulator and extended in a second direction that issubstantially perpendicular to said first direction; and a plurality ofmemory cells disposed at nodes of respective ones of said assist gatesand said plurality of word lines, wherein said memory device has amemory array structure in which a second conductive type inversion layeris formed electrically on a surface of said semiconductor substratelocated in the lower part of said assist gates, wherein said inversionlayer comprises a wiring for connection between adjacent ones of saidplurality of memory cells when a voltage is applied to said assistgates, wherein adjacent ones of said plurality of word lines areseparated electrically from each other by a side wall spacer, whereinsaid side wall spacer comprises an insulator formed at sides at leastone of even-numbered and odd-numbered ones of said plurality of wordlines, and wherein the space between adjacent ones of said plurality ofword lines is less than or about equal to ½ of a width of said wordlines.
 2. The semiconductor memory device according to claim 1, whereina top face of each of the even-numbered or odd-numbered word lineshaving said side wall spacer, is covered by a third insulator, andwherein a height of the top face of each of said word lines not havingsaid side wall spacer is substantially equivalent to that of a top faceof said third insulator.
 3. The semiconductor memory device according toclaim 1, wherein said plurality of word lines are divided into groups,each of which comprises first to fourth word lines disposedrepetitively, wherein ends of said first to fourth word lines arearranged having said first word line longer than said second word line,said third word line longer than said second word line, and said fourthword line longer than said third word line, wherein a first contact holefor applying a voltage to said third word line is formed on said one endof said third word line, wherein a second contact hole for applying avoltage to said fourth word line is formed on said one end of saidfourth word line, wherein other ends of said first to fourth word linesare arranged having said second word line longer than said first wordline, said second word line longer than said third word line, and saidthird word line longer than said fourth word line, wherein a thirdcontact hole for applying a voltage to said first word line is formed onan other end of said first word line, and wherein a fourth contact holefor applying a voltage to said second word line is formed on an otherend of said second word line.
 4. The semiconductor memory deviceaccording to claim 1, wherein said plurality of word lines are formedhaving a set of said first to fourth word lines adjacent to each other,wherein one end of said first to fourth word lines is arranged havingsaid first word line longer than said second word line, said third wordline longer than said second word line, and said third word line longerthan said fourth word line, wherein a third contact hole for applying avoltage to said first word line is formed on one end of said first wordline, wherein a first contact hole for applying a voltage to said thirdword line is formed on one end of said third word line, wherein an otherend of said first to fourth word lines is arranged having said firstword line longer than said second word line, said third word line longerthan said second word line, and said third word line longer than saidfourth word line, wherein a fourth contact hole for applying a voltageto said second word line is formed on the other end of said second wordline, and wherein a second contact hole for applying a voltage to saidfourth word line is formed on the other end of said fourth word line. 5.The semiconductor memory device according to claim 1, wherein each ofsaid plurality of memory cells comprises charge storage means, whereininformation is stored in each of the memory cells based on a value of acurrent flowing in said second conductor type inversion layer, whereinthe current flowing changes according to an amount of charge stored insaid charge storage means.
 6. The semiconductor memory device accordingto claim 5, wherein said charge storage means comprises one selectedfrom a plurality of semiconductor microcrystal grains and metallicmicrocrystal grains, insulated from each other through an insulator. 7.The semiconductor memory device according to claim 5, wherein saidcharge storage means is formed by an insulator having a charge trappingfunction.
 8. The semiconductor memory device according to claim 1,wherein a voltage applied to said plurality of word lines or said assistgates changes according to a memory address of each of said plurality ofword lines.
 9. The semiconductor memory device according to claim 1,wherein each of said plurality of memory cells is a multi-bit storagememory cell. 10-13. (Cancelled)